This invention relates to environmental air conditioning systems (ECS), and more specifically to air cycle environmental air conditioning systems such as used on aircraft
Aircraft that fly at altitudes above that at which ambient air is suitable for the health and comfort of crew and passengers are often equipped with air cycle environmental air conditioning systems. Such system provide pressurized conditioned air or for cooling passengers, crew, and other aircraft systems and components. These air conditioning systems typically use high pressure air bleed from a turbine engine or auxiliary power unit (APU). In some cases high pressure air may be provided by an electrically-powered compressor. The high pressure compressed air fed into these systems typically is at a temperature and pressure far in excess of the temperature and pressure required for conditioned air to be supplied to the cockpit and passenger cabin, so it must be expanded and cooled by the air conditioning system before it can be discharged into the aircraft cabin as conditioned air.
Air cycle environment air conditioning systems can operate by extracting work (i.e., mechanical energy) from turbines that expand and cool the high pressure compressed air. This work may be used to power components of the air conditioning systems such as compressors or fans. However, the typical system design has limited efficiency because it requires a source of high pressure compressed air that is 15-20 psia (pounds per square inch, absolute) (103-138 kPa) over the needed air pressure needed to pressurize the cabin with conditioned air (Approximately, 14.7 psia (101 kPa)). Bleeding high pressure compressed air from aircraft engines adversely impacts aircraft fuel efficiency during flight. Therefore, it would be desirable to operate an ECS system without the need for high pressure compressed air. Additionally, ECS systems are sized to operate at the lowest expected available engine bleed pressure (i.e., the highest altitude at which the aircraft is expected to fly), resulting in wasted energy during portions of the flight profile.